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Hair dryer discovery leads to waterproof paper

Making paper more water-resistant while retaining biodegradability and ease of recycling could open the door to many more applications. A fortuitous discovery is pointing the way in the PHYCELLO project.

Cellulose-based paper and board products are extremely versatile in meeting societal needs from communication and education to personal hygiene and the protection of perishable foodstuffs and delicate goods. Manufactured from renewable resources, they are also inexpensive, biodegradable and easily recycled. Yet a notable barrier to more widespread use has been the difficulty in conferring adequate water resistance without compromising these natural qualities. And, despite considerable progress in recent years, the industry continues to face problems in achieving pollution-free production. A chance observation by French biochemist Daniel Samain could provide an answer to both of these problems.

New kind of chemistry

Samain's discovery began when, while working on cancer treatments at the Centre National de la Recherche Scientifique (CNRS) in Toulouse, France, he sought to amuse his children with a Christmas message written in invisible ink. He wrote 'Joyeux Noel' on a sheet of paper using a water-repellent fatty acid chloride – so that, when the sheet was immersed in water, the words would appear in dry contrast against the wetted background. In haste to finish the task, he heated the fatty acid chloride impregnated paper using a handily available hair dryer. But when the message was developed, he noted that the letter 'O' had become a completely filled dry circle.

Puzzled, he looked further into the phenomenon during his spare time, and deduced that this was not a simple mechanical spread. Rather, the hot air was acting as a solvent, dispersing the fatty acid chloride at a rate that could be controlled by varying the flow and temperature. As it advances, the reagent binds chemically onto the surface of the fibres to produce a covalently grafted monomolecular hydrophobic layer.

The effect is comparable to that exploited in the analytical technique of gas chromatography, so Samain describes it as 'chromatogenic' chemistry'. "My investigations showed the reaction was extremely rapid and the yield in terms of transfer to the paper substrate actually increased exponentially in proportion to the boiling point of the reagent," he recalls. "Thus, the rate of deposition of a 22-carbon acid chain was an incredible one million times faster than that of the 2-carbon acetic acid derivative! This was clearly the basis of a previously unremarked, but highly exploitable, technology – which I quickly patented."

Following several false starts and frustrations, he established the SME Tsai Lun – named after the Chinese inventor of paper – and sought partners with the complementary skills needed to form a consortium that could apply for European Commission support to bring the idea to industrial fruition. The proposal was eventually accepted, and the three-year PHYCELLO project commenced in April 2001.

The original partnership underwent some changes, including the eventual withdrawal of Tsai Lun itself. Overall co-ordination is now with the Ahlstrom paper group while Samain works in association with CNRS. Other research participants are the Institut National Polytechnique de Toulouse and the International University Bremen, joined by French chemicals group SNPE, Spanish paper machinery manufacturer Brunnschweiler, and the Banque de France – a potential end-user with a particular interest in extending the longevity of banknotes.

Cleaner technology

"There are distinct disadvantages to the ways in which industry currently seeks to make paper products more water repellent," Samain explains. "One route is to disperse rosin or other organic sizing agents in the mixture of water and fibre that is the starting point of the manufacturing process. This is inefficient, because the chemicals react with the water, causing high levels of waste and the formation of slimy degradation products that necessitate frequent cleaning of the machine. Alternatively, the paper is impregnated with wax or laminated with polymeric layers, both of which reduce its biodegradability and recyclability.

"The PHYCELLO process, on the other hand, needs no water, can be applied further down the process line, and is fast enough to be compatible with high-speed reel-to-reel production. It also uses very small quantities of chemicals and, with sufficient development effort, should become cost-effective enough for widespread use. The result is natural products that retain all the desirable attributes of sustainability and environment-friendliness."

Industrially applicable

Within the framework of the present project, the partners are exploring both off- and on-line treatment methods. The former is already demonstrable at pilot scale, but the latter poses greater challenges that will take longer to resolve.

"We can use conventional flexographic printing techniques to apply the chemicals, then pass them through an oven with an airflow that distributes them evenly over the substrate," Samain observes. "Our set-up can already run at speeds in excess of 300 m/min – and application can even take place after the printing stage, which is what would be needed to produce water- and grease-resistant banknotes, for example."

For incorporation into large-scale integrated papermaking lines, Brunnschweiler is working on introducing the waterproofing agent via an existing type of air-cushioned roll that is used to support the web before final drying. At this stage, the grafting reaction can take place without any adverse reaction due to the remaining 5 to 6% of water.

Although the fatty acid chlorides have toxic properties, effective containment systems are readily incorporated. The hydrochloric acid by-product of the reaction can also easily be neutralised – or could possibly be collected as a saleable by-product.

Versatile process

Hydrophobicity is not the only property that can be conferred to paper by the PHYCELLO process technology. Ahlstrom, for example, is specifically aiming to treat the backing layer of self-adhesive label stock to obtain more effective bonding of the silicone release coating. Still more possibilities can be envisaged with the formulation of molecules exhibiting appropriate functionalities.

Nor is cellulose fibre the only substrate that can be treated. Even before the start of PHYCELLO, Samain had made significant progress in developing a durable water-shedding coating for the glass of automotive windscreens.

Chromatogenic chemistry could herald a whole new field of process technology.